powerpc/mm: Change setbat() to take a pgprot_t rather than flags

The callers of setbat() are actually passing a pgprot_t for the flags
parameter. This doesn't matter unless STRICT_MM_TYPECHECKS is enabled.
So we can turn that on without breaking the build, change setbat() to
take a pgprot_t and have it convert it to an unsigned long internally.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>